A Dyson sphere is a hypothesized artificial habitat built around a star by a civilization with sufficient technology. It is intended to capture as much as possible of the power output of the star, and should be a distinctive characteristic of a civilization able to control the resources of a planetary system.

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Search for alien civilizations

Giuseppe Cocconi and Philip Morrison, who were at the time professors of physics at Cornell University, proposed in 1959 that radio-astronomers should search for artificial radio-signals transmitted by alien civilizations (Cocconi et al., 1959). Such signals might be aimed at us by the aliens, or might be sent by one alien civilization to another and overheard by us. Cocconi and Morrison pointed out that narrow-band radio signals transmitted by existing radars and received by existing telescopes could be detected over interstellar distances, so that a search for such signals was reasonable. Frank Drake quickly began a search which he called "Project Ozma", using the 85-foot-diameter radio-telescope at Green Bank (Drake, 1959). He wrote, "It is named for the queen of the imaginary land of Oz, a place very far away, difficult to reach, and populated by exotic beings". He did not hear any signals. Searches for alien radio signals have continued ever since, with increasing sensitivity and increasing coverage of the sky, but always with negative results. It appears that alien civilizations, if they exist in our neighborhood, do not wish to communicate by radio.

Dyson in 1960 asked the question, whether there was some way to detect alien civilizations who did not wish to communicate, and answered it in the affirmative (Dyson, 1960). He observed that any civilization using a large flow of energy, whether or not it wished to communicate, would be forced by the laws of thermodynamics to get rid of waste heat, which must be radiated into space in the form of infra-red radiation. Uncommunicative civilizations could therefore be detected as sources of infra-red radiation, provided that they used a flow of energy large compared with natural infra-red sources in the same part of the sky. Dyson proposed that a search be made for point sources of radiation in the ten-micron band. Ten-micron radiation would be emitted from the outer surface of any habitat containing creatures at a temperature around 300 degrees Kelvin at which water is liquid. Because the earth's atmosphere radiates strongly in this band, a search for extraterrestrial sources requires a telescope with an infra-red detector orbiting in space. Interestingly, at the time of Dyson’s proposal infra-red astronomy in the ten-micron region was mostly a wild gleam in the eyes of a few mavericks. It has since become one of the dominant windows on the sky.

According to the Russian astrophysicist Nikolai Kardashev (Kardashev, 1962), long-lived civilizations are likely to belong to three types which he calls I, II and III. Type I controls the resources of a planet, type II controls the resources of a star, and type III controls the resources of a galaxy. Our own civilization is not yet type I but will be so within a few hundred years at most. A Dyson sphere is an example of a type II civilization. Kardashev observed that the transitions from birth of a civilization to type I, from type I to type II, and from type II to type III, will be rapid compared with the probable life-times of civilizations that remain in one of the three types. A type I civilization at interstellar distance will be undetectable with existing instruments, and a type III in our own galaxy would change the appearance of the sky so drastically that it could hardly have escaped our attention. So any alien civilization that we discover is likely to be a type II civilization controlling the resources of a star. The civilization would occupy a habitat surrounding the star and using for its own purposes the star's entire output of starlight. The flux of energy carried by the starlight would then be radiated as waste heat from the outer surface of the habitat. From the perspective of the twenty-first century a Dyson sphere built from the shards of Jupiter would be a big project, a factor of ten billion trillion times heavier than the International Space Station. Dyson’s paper (Dyson, 1960) proposing the search for artificial infra-red sources notes that it might take “the energy radiated by the sun in 800 years” to build one.

In his paper Dyson wrote, "Taking our own solar system as the model, we shall reach at least a possible picture of what may be expected to happen elsewhere. I do not argue that this is what will happen in our system; I only say that this is what may have happened in other systems". He remarked that the mass of Jupiter, if distributed in a spherical shell revolving around the sun at twice the earth's distance from it, would have a thickness of 2 tons per square meter of surface area. "A shell of this thickness", he wrote, "could be made comfortably habitable, and could contain all the machinery required for exploiting the solar radiation falling onto it from the inside". This remark gave to readers the misleading impression that the habitat of an alien civilization would be a big round ball with a star at the center. Various science-fiction writers adopted this notion of a big round ball inhabited by aliens and gave it the name "Dyson Sphere". Dyson used the phrase "artificial biosphere" to describe the habitat of an alien civilization. He was well aware that the artificial biosphere could not be a big round ball. A big round ball, whether rotating or not, would be mechanically too weak to support its own weight against the gravity of the star. He imagined the artificial biosphere to be a cloud of inhabited objects orbiting a star, surrounding the star densely enough to absorb all the starlight, but with the orbits carefully arranged so as to avoid collisions.

Astronomical searches

After Dyson's proposal, twenty years went by before a spacecraft carrying a liquid-helium-cooled detector of long-wave infra-red radiation was launched. In 1983 the Infra-red Astronomy Satellite (IRAS), an international mission funded jointly by the United States, the United Kingdom and the Netherlands, did the first sky-survey in the long-wave infra-red band, discovering 245389 sources (Beichman, 1987). The mission was spectacularly successful. It discovered that the sky is infested with infra-red sources. The vast majority of the sources are identifiable natural objects. Many are new-born stars which are still embedded in the dense dust-clouds out of which they recently condensed. The dust-clouds are heated by the new-born stars inside them, and radiate away the energy of the stars in the form of infra-red radiation. As seen by our infra-red detectors, these natural "cocoon stars" look just like alien civilizations. Old stars such as carbon stars going off the Asymptotic Giant Branch also blow off large dust clouds. If any alien civilizations exist in our neighborhood, they are hidden among the huge population of natural infra-red sources.

Since most of the infra-red sources have reasonable astrophysical explanations Ockham’s famous razor plays an important role – we should use the simplest explanation. Still, in spite of the challenge of natural Dyson sphere look-alikes scientists such as Carl Sagan and Jill Tarter, Director of the Center for SETI Research at the SETI Institute, tried to find the Dyson sphere needles in the infra-red haystack. Recently Dick Carrigan (Carrigan, 2009) has used information from the IRAS low resolution spectrometer (LRS) to build on earlier IRAS filter-based searches by several Russian astronomers including Kardashev and Viacheslav Slysh. Carrigan has found that only a handful of the IRAS sources come close to passing muster as a Dyson sphere. Many sources have clear spectrographic lines that identify them as mimics. Part of Carrigan’s toolkit is fitting to a Planck blackbody distribution. That is a step beyond but also a step down from Dyson’s original and more general perspective. In practice, most of the LRS spectra don’t look much like a Dyson sphere and have higher temperatures. The search suggests that there are few if any even mildly interesting candidates within several hundred light years of earth.

Several other astronomers including Dan Werthimer, famous as the originator of the SETI@home program and Jun Jugaku have looked for so-called “partial” Dyson spheres where the star is only partly obscured. They have carried out the searches by looking for infra-red excesses around visible stars. No likely candidates were reported in searches of several thousand stars.

After IRAS, other spacecraft have been launched, carrying more sensitive detectors and discovering an even larger number of sources. The Spitzer Space Telescopeis the best of the breed with much better resolution and sensitivity than IRAS. In most cases the mission has focused hard on investigating natural sources in more detail. One part of the mission, GLIMPSE (Galactic Legacy Infrared Mid-Plane Survey Extraordinaire), has used filters to investigate infra-red sources in the galactic plane. Ed Churchwell from the University of Wisconsin reports that GLIMPSE has identified on the order of 100 million sources, 90% of which are cool red giants. Unfortunately all of the Spitzer filters are at shorter wavelengths so the sample population is biased toward temperatures above the natural range for life. In order to continue the search for Dyson spheres the problem of discriminating artificial from natural sources must continue to be faced. Until there is excellent discrimination with few false alarms, increasing the sensitivity of detectors will not help.

Literary origin

The true progenitor of the idea of an artificial biosphere was the science-fiction writer Olaf Stapledon. In his novel, "Star Maker" (Stapledon, 1987, page 179), he described the appearance of a galaxy in the far future occupied by a type III civilization and seen from a great distance. "Not only was every solar system now surrounded by a gauze of light traps, which focused the escaping solar energy for intelligent use, so that the whole galaxy was dimmed, but many stars that were not suited to be suns were disintegrated, and rifled of their prodigious stores of sub-atomic energy". The novel was published in 1937 and Dyson read it in 1945. Unfortunately he neglected to refer to Stapledon when he published his paper (Dyson, 1960). If Dyson had given proper credit to Stapledon in that paper, then the artificial biospheres would have been called "Stapledon clouds", and would never have been given the misleading name "Dyson spheres".

The junior author feels Dyson is too modest. Modern SETI (Tarter, 2001) rests on four great pillars from the middle years of the twentieth century. These are the Fermi paradox – a patient space traveler can go a long way traveling at 1/100 of the speed of light so why hasn’t he appeared here, Cocconi and Morrisons’ recognition that interstellar radio is possible, the Drake equation, and Dyson’s suggestion. The spirit of Dyson’s idea, look for pyramids rather than radio signals, may ultimately be the paradigm that identifies intelligence beyond the solar system.

References

C. Beichman, "The IRAS View of the Galaxy and the Solar System", Annual Reviews of Astronomy and Astrophysics, 25, 521-563 (1987).